Hi I have found this audio-pde at http://www.ee.columbia.edu/~dpwe/resources/Processing/.

As I have no clue on how to learn about audio-filters elsewhere, I am trying to learn from that code, by editing it, and merging it in to something more useful.

Processing.org has this example on how to open a file for 'processing it'  : http://www.processing.org/reference/selectInput_.html

As I try to merge the two (as followed):

import ddf.minim.analysis.*;
import ddf.minim.*;
 
Minim minim;
AudioPlayer sound;
FFT fft;
Filter filter;

boolean paused = false;

// spectrum display configuration
int buffer_size = 2048;  // also sets FFT size (frequency resolution)
float gain = 40; // in dB
float dB_scale = 2.0;  // pixels per dB

int spectrum_height = 200; // determines range of dB shown
int legend_height = 20;
int spectrum_width = 512; // determines how much of spectrum we see
int legend_width = 50;

// filter properties
float centerFreq = 1000;
float bandwidth = 500;

String fselR = "";


void setup()
{
 
  try{
  selectInput("Select a file to process:");
  }
  catch(Exception e){ 
  }
 
  // size(legend_width+spectrum_width, spectrum_height+legend_height, P2D);
  // size has to be explicit for the applet export to extract it correctly
  size(562, 220, P2D);
  //textMode(SCREEN);
  textFont(createFont("SanSerif", 12));
 
  minim = new Minim(this);
 
  sound = minim.loadFile(fselR, buffer_size);
 // sound = minim.loadFile("sound.mp3", buffer_size);
  // make it repeat
  sound.loop();
 
  // create an FFT object that has a time-domain buffer
  // the same size as line-in's sample buffer
  fft = new FFT(sound.bufferSize(), sound.sampleRate());
  // Tapered window important for log-domain display
  fft.window(FFT.HAMMING);
 
  // create a filter and insert it
  filter = new Filter(centerFreq, bandwidth, sound.sampleRate());
  sound.addEffect(filter);
}

// Convert a spectrum value into a y pixel position
int spectrum_y(float bandval)
{
  float val;
  if (bandval > 0) {
    val = dB_scale*(20*((float)Math.log10(bandval)) + gain);
  } else {
    val = -200;  // avoid log(0)
  }
  int y = spectrum_height - Math.round(val);
  if (y > spectrum_height) { y = spectrum_height; }
  return y;
}

String fileSelected(File selection, String fses) {
  try{
  if (selection == null) {
    println("Window was closed or the user hit cancel.");
  } else {
    println("User selected " + selection.getAbsolutePath());
  fselR = selection.getAbsolutePath();
  }
  }
  catch(Exception e){
 fselR = "";
  }
return fselR;
}

void draw()
{
etc...

- - -

(the italic lines: is my own merging code)

The compiler complains about "GLExeption...: cannot call invokeAndWait..." as if the main audio-example won't permit me to have a pause in execution, where selected input of a pickable *.mp3 is put into the system as a string varible (to be used at: sound = minim.loadFile(fselR, buffer_size);)